It's also going to be susceptible to some "popularity contest" skew. For my part, I've been known to post the sample code for my questions in C# even if that's not actually the language I'm working in when I hit the problem. C# is the lingua franca of the platform, so the questions is likely to be seen by a wider audience that way.

That said, I've wondered for a long time if the Tiobe index is subject to a potentially much bigger source of skew - namely, I suspect that C, C++ , and Java get a boost for being three of the most popular languages for people who spend more time talking about programming than actually programming.

Additionally, AFAIK, stackoverflow started out as .net/C# community, so they're probably somewhat biased towards that. To remove such bias, you'd probably need to analyze at least a few dozen such forums.